1. Field of the Invention
The present invention relates to a tire monitoring system which can receive the information on a tire condition such as tire pressure transmitted from wireless-type tire sensors mounted on each tire using a receiver for a keyless entry system and inform the abnormality of a tire such as a puncture.
2. Description of the Prior Art
Japanese Unexamined Patent Publication No. Sho 62-87816 (1987) discloses a tire pressure related information display device in which a tire sensor for detecting tire pressure (i.e., tire air pressure) is mounted on each tire for radio transmission. The display device not only judges whether there is a puncture based on the detected tire pressure to display generation of the puncture, but also displays the distance in which the punctured tire can run.
Japanese Unexamined Patent Publication No. Hei 3-135810 (1991) discloses a tire pressure abnormality detecting method for preventing wrong detection of a puncture by tire pressure punctuation due to allotted or distributed load which varies with lateral acceleration or cross acceleration affecting a car body when a car is turned or suddenly braked. In this method, a moving amount of load is estimated in response to the acceleration detected by an acceleration sensor and the allotted or distributed load of each tire is corrected by the estimated moving amount of load. Then, an average tire pressure of each tire is set according to the allotted or distributed load corrected above. The detected tire pressure of each tire is compared with the average tire pressure serving as a reference tire pressure to detect abnormality of the tire pressure.
Japanese Unexamined Patent Publication No. 2000-16037 discloses a tire pressure information transmitting and receiving device in which the change of tire pressure after abnormality of the tire pressure has been generated can be detected on the side of a receiving device while reducing electric power consumption.
This tire pressure information transmitting and receiving device is construed as follows. Namely, a transmitting device transmits a tire pressure abnormal signal when abnormality of the tire pressure is detected. The transmitting device subsequently transmits the tire pressure abnormal signal and a tire pressure normal signal, respectively, only when there is caused a change of the tire pressure over a predetermined value and when the tire pressure returns to its normal condition. The transmitting device also transmits the tire pressure normal signal at regular hour intervals when the tire pressure is normal. A receiving device lights a first warning lamp when the tire pressure abnormal signal is received and lights a second warning lamp when any signal is not received over the regular hour.
Japanese Unexamined Patent Publication No. 2000-118214 discloses a tire internal pressure-monitoring device which can realize monitoring of the tire internal pressure for a long time while controlling electric power consumption.
This tire internal pressure-monitoring device is constructed as follows. Namely, when the tire internal pressure drops to a certain pressure level, a tire internal pressure detector closes a current-carrying switch to supply a coded information generating section and a telemeter transmitter with electricity from a power supply battery. Thus, a tire breakage danger signal is transmitted from a transmitting antenna. A receiving section receives the danger signal and then informs by an alarm that there is some possibility that the tire will be punctured.
Described in Japanese Unexamined Patent Publication No. 2000-289418 is a power supply unit with a built-in tire pressure sensor. In this power supply unit, the tire pressure sensor is provided inside a car tire and a battery is secured to the outside of the tire so that the battery can be directly detached and exchanged outside the car tire.
Japanese Unexamined Patent Publication No. 2000-355203 discloses a tire pressure warning device which detects abnormality of the tire pressure and informs it to a car body side by wireless. In this warning device, abnormality of the tire pressure can be immediately informed while holding the electric power consumption for detecting and informing the abnormality to the minimum.
This tire pressure warning device is constructed as follows. Namely, a tire-side warning device installed inside a tire is provided to allow a control circuit to actuate when a vibration sensor detects micro-vibration of the tire. The control circuit detects the pressure and temperature inside the tire by driving a pressure sensor and a temperature sensor and judges whether they are normal or abnormal. When they are abnormal, a transmitting circuit is driven to transmit a signal for showing the abnormal contents to a car body-side warning device. Since the control circuit stops its operation when the car is parked or stopped in which the tire does not vibrate, it is possible to prevent the battery consumption.
In such systems as to detect the abnormality of a tire to generate an alarm or the like, it is necessary to provide on a tire side a sensor for detecting a tire condition such as air pressure of the tire and temperature of the tire and a transmitter for radio-transmitting the information on the detected tire condition. It is also necessary to provide a receiver on a car body side.
Further, a car with a keyless entry system that can remotely control locking or unlocking of a door has come into wide use recently. In the case where the car with such a keyless entry system is further provided with a tire monitoring system, it is necessary to provide a receiver for the tire monitoring system as well as a receiver for the keyless entry system. In this case, it is not only economical, but also there is needed a space for installing the receivers.
On the other hand, the tire pressure also varies with an outside air temperature. A car tire is also heated by friction between itself and the road surface during running. Air inside the tire expands with this heating and the internal pressure remarkably fluctuates. Further, the heating varies in the case where a car is running at a constant speed, the speed is increased, the speed is reduced, the car is running in a city area, or it is running at high speed. Accordingly, in a conventional tire puncture detecting technique whereby a tire puncture is detected only based on the tire pressure, there is some possibility that the tire will be detected as punctured even in the case where the tire is not punctured.
Further, in such system as to detect abnormality of the tire to generate the alarm or the like, the tire pressure or the tire abnormal information is transmitted from the wireless-type tire sensor provided on each tire. In this case, it is not possible to judge which tire will be punctured. There is also some possibility that a wireless signal transmitted from a tire of the other adjacently running car will be received.